This invention relates to a method of measuring thermal conductivity. More specifically this invention is directed to a rapid, non-destructive technique of measuring the thermal conductivity of various materials, including solids, powders and liquids.
Thermal conductivity measurement is a very important part of material analysis in industries ranging from electronics to construction. Materials with high conductivity for example, may be used in electronics applications as heat sinks to dissipate heat away from sensitive components. In the construction industry, low conductivity is a requirement in building materials where insulation is an important consideration.
Thermal property measurement techniques include either the transient or steady-state instrumentation categories. In steady-state measurements, heat is applied to a sample until a constant temperature equilibrium is reached, while the transient method involves applying heat to a sample over a period of time and measuring the changing temperature response of the sample. There are a number of instruments available to measure thermal conductivity using either the transient or steady-state methods. Some of these instruments include guarded hotplate, hot wire, modified hot wire, laser flash and transient plane source.
Guarded hotplate is a steady-state technique that involves placing a solid sample of fixed dimensions between two temperature controlled plates at different temperatures. Typically, one plate is heated while the other is cooled and the temperatures of the plates are monitored over time, until they reach constant temperatures (xcex94T). The steady-state temperature, sample thickness (L) and area (A) as well as the heat input (Q) to the system are used to calculate the thermal conductivity (k) from Q=kA/Lxcex94T. While steady-state methods are generally very accurate, they are also time consuming, taking hours to complete a single test.
The hot wire technique is a transient method of determining thermal conductivity. This means that the temperature rise is measured during a time interval. The method involves inserting an electrically heated wire into a sample. The heat flows out radially from the wire and the temperature of the wire is measured. The plot of these temperatures versus the logarithm of time is used to calculate thermal conductivity. This technique is intrusive and therefore has limitations as to the types of materials that can be tested. (ie. powder and liquid but not solids)
Another method of measuring thermal conductivity is the transient plane source method also called the hot disk method. This is another transient technique in which the sample surrounds a heating element, but in this case, the sensor is configured as a planar circle rather than a wire or line source. The heating of the element causes a three dimensional heat flow to occur. The interface temperature is monitored and plotted against a time function, and thermal conductivity and diffusivity are calculated from an iterative curve fit to the underlying equations. This method does not require density or heat capacity information. While this method is useful for certain types of samples such as liquids or powders, the measurement of solid material requires that 2 identical sized samples of the same material be used to obtain 1 thermal conductivity measurement.
Modified hot-wire techniques also exist in the art. The modification to the basic hot wire design is that the heating element is supported on a backing material in the sensor and as such, the heat flows both into a sample to be measured and into the backing material. Since the heat flows into two different materials, the effect of the backing material must be accounted for through calibration. In order to measure thermal conductivity using a non-destructive modified transient hot wire technique, a suitable probe such as that provided for U.S. Pat. No. 5,795,064, issued Aug. 18, 1998 to Mathis, may be used, the entire disclosure of which is hereby incorporated by reference. While non-destructive due to the surface measurement and interfacial nature of the sensor interaction with the sample, they generally do not provide a direct measurement of the thermal conductivity. The primary result is effusivity ((kpcp)xe2x80x94the square root of thermal conductivity, density and heat capacity), requiring that the heat capacity and density parameters of the sample must be known to calculate thermal conductivity from the thermal effusivity.
Another transient technique is the laser flash diffusivity method. This method involves applying a short pulse of heat to the front face of a specimen using a laser, and measuring the temperature change of the rear face with an infrared (IR) detector. The resulting temperature rise of the rear face of the test specimen is monitored as a function of time and used, together with the sample thickness, to determine the thermal diffusivity. While this method rapidly determines diffusivity, the result must be combined with density and heat capacity data to calculate thermal conductivity.
What is needed is a rapid, non-destructive technique of measuring thermal conductivity of materials of any configuration, in which the density and heat capacity do not need to be supplied.
An object of the present invention is to provide a method and system to measure the thermal conductivity of a solid or fluid sample, rapidly and non-destructively. It is a further objective of this invention to provide a measurement technique that does not require heat capacity or density information of the sample to be known in order to determine the thermal conductivity. The present invention further provides the ability to measure both oriented materials and non-homogeneous materials.
The invention preferably involves using a modified hot wire technique to rapidly measure the thermal conductivity of a sample of interest. The disclosed method and system accomplish this by applying backing material within a sensor to support the heating element of the sensor. In this way the sensor system behaves similarly to an intrusive probe in which the sensor is completely surrounded by a material, but in this case, the material is a composite of the unknown sample and the backing material. The instrument response of the sensor corresponds to this combination of materials surrounding the heating element. The instrument is calibrated to cancel the effects of the backing material out of the instrument response.
In accordance with an aspect of the invention, there is provided a method for calibrating a sensor instrument for measuring thermal conductivity. The method comprises steps of (a) bringing a heating element supported on a backing material in contact with a known material having a known thermal conductivity such that the heating element is substantially surrounded by the known material and the backing material; (b) supplying a heat via the heating element to the combination of the backing and known material; (c) monitoring a temperature increase to obtain a raw instrument response over a predetermined time period; (d) determining, from the raw instrument response, a relation parameter indicative of the relation between the temperature increase and the time; (e) analyzing an adjusted instrument response by compensating the raw instrument response based on the relation parameter and an adjusting factor while iteratively changing the adjusting factor; (f) determining the adjusting factor as a calibration factor when the adjusted instrument response reaches predetermined agreement to the known thermal conductivity of the known material; and (g) calibrating the sensor instrument using the calibration factor.
In accordance with another aspect of the invention, there is provided a method for measuring a thermal conductivity of a material. The method comprises the steps of (1) calibrating a sensor instrument by the steps of; (a) bringing a sensor having a backing material and a heating element in contact with a known material having a known thermal conductivity such that the heating element is substantially surrounded by the known material and the backing material; (b) supplying a heat to the known material; (c) monitoring a temperature increase to obtain a raw instrument response over a predetermined time period; (d) determining, from the raw instrument response, a relation parameter indicative of the relation between the temperature increase and the time; (e) analyzing an adjusted instrument response by compensating the raw instrument response based on the relation parameter and an adjusting factor while iteratively changing the adjusting factor; and (f) determining the adjusting factor as a calibration factor when the adjusted instrument response reaches predetermined agreement to the known thermal conductivity of the known material. The method further comprises the steps of (2) bringing an unknown material to be tested in contact with the sensor such that the heating element is substantially surrounded by the unknown material and the backing material; (3) obtaining an instrument response by supplying a heat to the unknown material and monitoring a temperature increase by the sensor; and (4) obtaining the thermal conductivity of the unknown material from the instrument response based on the calibration factor.
In accordance with another aspect of the invention, there is provided a system for calibrating a sensor instrument for measuring thermal conductivity. The system comprises an instrument response receiver for receiving, from a sensor having a heating element and a backing material, a raw instrument response representing a temperature increase in a known material having a known thermal conductivity when a heat is supplied to the known material, the heating element is substantially surrounded by the known material and the backing material; a relation parameter analyzer for determining, from the raw instrument response, a relation parameter indicative of the relation between the temperature increase and the time; a calibration factor finder for analyzing an adjusted instrument response by compensating the raw instrument response based on the relation parameter and an adjusting factor while iteratively changing the adjusting factor, the instrument response analyzer determining the adjusting factor as a calibration factor when the adjusted instrument response reaches predetermined agreement to the known thermal conductivity of the known material; and a calibrator for calibrating the sensor instrument using the calibration factor.
In accordance with another aspect of the invention, there is provided an instrument for measuring thermal conductivity of a material. The instrument comprises a heating element for supplying a heat to a test material to be measured; a detector for measuring instrument responses by monitoring a temperature increase in the test material; a backing material for surrounding the heating element with the test material; and an instrument response analyzer. The instrument response analyzer has a calibration factor determiner for receiving a first instrument response when the test material is a known material having a known thermal conductivity, the calibration factor determiner further determining a relation parameter indicative of the relation between the temperature increase and the time based on the first instrument response, and analyzing an adjusted instrument response by compensating the raw instrument response based on the relation parameter and an adjusting factor while iteratively changing the adjusting factor, the instrument response analyzer determining the adjusting factor as a calibration factor when the adjusted instrument response reaches predetermined agreement to the known thermal conductivity of the known material; and a compensator for receiving a second instrument response when the test material is an unknown material having an unknown thermal conductivity, the compensator determining the thermal conductivity of the unknown material from the second instrument response based on the calibration factor.
In accordance with another aspect of the invention, there is provided a system for measuring thermal conductivity of a material. The system comprises an instrument response receiver for receiving from a sensor raw instrument responses when a heat is supplied to a test material, the sensor having a heating element and a backing material for surrounding the heating element with the test material; and an instrument response analyzer having a calibration factor determiner for receiving a first instrument response when the test material is a known material having a known thermal conductivity, the calibration factor determiner further determining a relation parameter indicative of the relation between the temperature increase and the time based on the first instrument response, and analyzing an adjusted instrument response by compensating the raw instrument response based on the relation parameter and an adjusting factor while iteratively changing the adjusting factor, the instrument response analyzer determining the adjusting factor as a calibration factor when the adjusted instrument response reaches predetermined agreement to the known thermal conductivity of the known material; and a compensator for receiving a second instrument response when the test material is an unknown material having an unknown thermal conductivity, the compensator determining the thermal conductivity of the unknown material from the second instrument response based on the calibration factor.
In accordance with another aspect of the invention, there is provided a computer readable medium storing the instructions or statements for use in the execution in a computer of the method for calibrating a sensor instrument for measuring thermal conductivity.
In accordance with another aspect of the invention, there is provided electronic signals for use in the execution in a computer of the method for calibrating a sensor instrument for measuring thermal conductivity